Sulfur dioxide (SO2) is an unwanted by-product of many combustion processes. SO2 is harmful to the environment as it produces acid rain when mixed with the water and oxygen present in the earth's atmosphere. Some of the most widespread sources of SO2 include emissions from diesel engines and electrical power plants run by fossil fuels. Accordingly, there is increasing pressure to regulate and control the levels of SO2 emanating from such sources, therefore monitoring of SO2 levels is required.
All of the commonly-used devices for detecting SO2 are limited to operating at maximum temperatures no more than about 40° C. An example of such a detector is the SO2-AF sensor from Alphasense Ltd. This sensor requires operation within the temperature range of −30 to 40° C. Other types of detectors used in the art include, for example, the electrolytic conductivity detector, mass spectrometer, and gas chromatography detector. Other popular detection methods are based on, for example, thermal conductivity and flame ionization. All of these detection methods are designed to operate more preferably at or near room temperature.
This upper limit on the operating temperature necessitates cooling of the combustion exhaust before measurement of the SO2 level can take place. Cooling of the sample, and typically other pretreatment steps (such as scrubbing of hydrocarbons), can alter the sample by, for example, contamination, dilution or loss of sample species, or conversion of the SO2 into sulfur trioxide (SO3). In addition, the time required before the sample can be measured increases the chances that active species can decompose or react with other species. Furthermore, cooling of the sample causes condensation. The condensation is problematic because it interferes with the measurement process.
In addition to the difficulties caused by cooling, some of the devices currently used to measure SO2 are not real-time in nature. For example, the electrolytic conductivity detector requires a gas sample to be absorbed by a Liquid electrolyte. These detectors operate by measuring the difference in electrical conductivity of the electrolyte material before and after the gas sample is absorbed by the electrolyte. The difference in electrical conductivity of the electrolyte indicates the type and/or concentration of the gas. See, for example, U.S. Pat. No. 4,440,726.
The absorption step required by the electrolytic conductivity detector has the disadvantage of increasing the measurement time. The long sample preparation time is particularly problematic in situations where SO2 is needed to be monitored over time, continuously or in intervals. In such cases, extrapolation of a result back in time is required. The extrapolation can contribute to inaccuracy of the results as well as to an uncertainty of when a certain SO2 level occurred.
There remains a need in the art for a simple and accurate method for directly measuring the level of SO2 emanating from a combustion source. To improve the accuracy and usefulness of the measurement, there remains a particular need for a method that can determine in a real-time fashion the levels of SO2 at the high temperatures encountered near combustion events.